Seasonal changes in the diet of a critically endangered seabird and the importance of trawling discards
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Pelagic seabirds obtain food from oceans where the availability of their prey changes rapidly both seasonally and spatially. Here, we investigated changes in the trophic habits of the critically endangered Balearic shearwater (Puffinus mauretanicus) through the breeding season and tested for dietary differences between sexes and age classes. We analysed δ15N and δ13C values in blood of adults during the pre-incubation, incubation and chick-rearing periods and of their chicks. Using a two-isotope mixing model, we estimated dietary contributions based on isotope values from potential prey species which included small pelagic species available naturally and demersal fish species available only from trawling discards. Balearic shearwaters showed clear isotopic and dietary variation through the breeding season. During pre-incubation, breeding adults appeared to exploit demersal fish, whereas during the incubation and chick-rearing period, they fed mainly on pelagic anchovies (Engraulis encrasicolus) and pilchards (Sardina pilchardus). Similarly, chicks were fed mainly with anchovies, a resource with a high energetic value. This variation in the dietary habits of adult shearwaters during the breeding season was probably related to both natural and fishery-induced seasonal changes in the availability of potential prey species within their main feeding grounds. However, changes in the nutritional requirements of the shearwaters could also play an important role. Indeed, diet differed between sexes during pre-incubation: females fed less on trawling discards and more on small pelagic fish than males. This sexual segregation in diet could be the consequence of higher nutritional requirements of females during this period. Our study reveals the differential importance of both trawling discards and small pelagic fish species for a pelagic seabird depending on the breeding period and illustrates the importance of considering the entire breeding season when making inferences about the importance of specific prey in seabird dietary studies.
KeywordsBreeding Season Pelagic Fish Potential Prey Demersal Fish Ebro Delta
We extend thanks to Isabel Afán and to Arsenio Granados for their fieldwork and stable isotope analyses, respectively. Fish samples were collected within the frame of the MEDITS project, with the help of Luis Gil de Sola, Pere Abelló and the scientists and crew at R/V Cornide de Saavedra (Spanish Institute of Oceanography, IEO). Research funds were provided by several grants from the Spanish Government (P06-RNM-02362, REN2002-00450, BOS2003-01960 and CGL2006-04325/BOS). Maite Louzao was supported by a PhD fellowship of the Balearic Government and a post-doctoral fellowship of the Spanish Ministry of Education and Science (Ref. EX-2007-1148). Meritxel Genovart was funded by an I3P post-doctoral fellowship from the Spanish Ministry of Education and Science. Four referees contributed to improve a previous draft.
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